Impairment of induction of cytochrome P-450 by phenobarbital has been shown in selenium-deficient rats. Recent findings indicate accelerated heme catabolism in selenium-deficient phenobarbital-treated livers. The turnover of cytochrome P-450 under these conditions is being studied. Other aspects of heme synthesis and degradation are being studied in selenium deficiency. Recent unpublished studies have revealed that selenium-deficient rats failed to accumulate administered inorganic mercury in their kidneys and liver to the extent found in controls. Current studies are focusing on the effect of selenium status on the binding of selenium to various kidney and liver proteins.